Method for detecting pci collisions

ABSTRACT

A computer implemented method of operating a base station of a wireless communication network, wherein the method comprises: comparing a number of missing context release messages ( 108 ) with a first criterion; comparing the error rate of connection reestablishment messages ( 112 ) with a second criterion; executing a cell global identity search algorithm ( 110 ) if at least one of the first and the second criterion is fulfilled, wherein the cell global identity of at least one cell with the physical cell identity of the cell where the criterion was fulfilled is requested via an automatic neighbour relation measurement, and wherein a conflict of physical cell identities is detected and reported to an information algorithm ( 116 ) when for the physical cell identity at least two cell global identities are found; an information algorithm for informing a central network element ( 118 ) and/or at least one base station ( 120 ) of the mobile communication network about a conflict of physical cell identities when a conflict of physical cell identities is reported to the information algorithm; and initiating a reassignment of at least one physical cell identity.

FIELD OF THE INVENTION

The present invention relates to a method of detecting a conflict ofphysical cell identities in wireless communication networks.

BACKGROUND AND RELATED ART

In wireless communication networks, e.g. 3GPP Long Term Evolutionnetworks, every communication cell has a physical cell identity (PCI)and a cell global identity (CGI). While each CGI is dedicated to onesingle cell the number of PCIs being assigned to wireless communicationnetwork cells is limited to 504. So it cannot be excluded that twoidentical PCIs are selected for two different cells belonging to basestations of the wireless communication network being direct neighbors orbeing located in a short distance.

When one PCI is assigned to two different cells of the wirelesscommunication network that are direct neighbors or located in a shortdistance, the communication of mobile devices with the wirelesscommunication network in the proximity of the two cells with the samePCI can be disturbed.

It is therefore an object of the present invention to provide a methodfor avoiding conflicts of physical cell identities of different mobilecommunication cells.

SUMMARY OF THE INVENTION

This object is solved by the feature of claim 1. Preferred embodimentsof the invention are described by the features of the dependent claims.

The present invention provides a computer implemented method ofoperating a base station of a wireless communication network. Foravoiding conflicts of physical cell identities of different mobilecommunication cells a plurality of steps is performed as follows:

The number of missing context release messages during a handoverprocedure is compared with a first criterion. Usually, in the standardof 3GPP Long Term Evolution networks the context release message is a UE(User Equipment) context release message, the UE being preferably amobile device. Preferably, the first criterion is a first threshold ofmissing context release messages per a time interval. This isadvantageous for detecting if two different cells have the same PCIbecause during a handover procedure from a serving cell with a differentPCI to a first neighbour cell, that is prepared for the handover, havingthe same PCI as a second neighbour cell, that is not prepared for thehandover, the affected mobile device can try to execute a handover tothe second neighbour cell because of the PCI conflict so that thecontext release message from the first neighbour cell is missing in theserving cell.

Cell Global Identities (CGIs) are in 3GPP Long Term Evolution networksunique identifiers for each radio cell of a wireless communicationnetwork, Physical Cell Identities (PCIs) are in 3GPP Long Term Evolutionnetworks non-unique identifiers for each radio cell of a wirelesscommunication network.

The error rate of connection re-establishment messages is compared witha second criterion. Preferably, the second criterion is a secondthreshold of the error rate of connection re-establishment messages.This is advantageous because by analyzing the error rate of connectionre-establishment messages a PCI conflict of two different cells can bedetected. A connection re-establishment message is sent to a cell whenthe handover procedure of a mobile device was not successful. Forexample, a handover procedure of a mobile device is initiated by aserving cell with a different PCI to a first neighbour cell, that isprepared for the handover, with the same PCI as a second neighbour cell,that is not prepared for the handover. Because of the PCI conflict thefirst cell may be prepared for the handover of the mobile device whilethe mobile device executes a handover to the second unprepared cell withthe same PCI as the first cell. Thus, the mobile device detects ahandover failure because the second cell is not prepared for thehandover. Then the mobile device sends a connection re-establishmentmessage to the second cell.

If at least one of the first and the second criterion is fulfilled thecell global identity search algorithm is executed. The algorithmrequests via an automatic neighbor relation measurement the cell globalidentity of at least one cell with the physical cell identity of thecell where the criterion was fulfilled. The automatic neighbor relationmeasurement is performed by at least one mobile device. The at least onemobile device reports the cell global identity of each cell with thephysical cell identity requested by the algorithm. If at least two cellswith different cell global identities and the same physical cellidentity are detected the conflict of physical cell identities isreported to an information algorithm.

The information algorithm informs a central network element and/or atleast one base station of the wireless communication network about theconflict of physical cell identities.

A reassignment of at least one physical cell identity is initiated.Preferably, one of the at least two base stations serving the at leasttwo cells with the same physical cell identity changes the physical cellidentity of the served cell by picking a new physical cell identity of arange of physical cell identities not including known physical cellidentities of cells being located near the cell. Another possibility isthat the central network element assigns a new physical cell identity tothe cell by avoiding physical cell identities of cells being locatednear the cell.

In accordance with an embodiment of the invention, the method furthercomprises receiving the physical cell identities and the cell globalidentities of the cells served by the neighboring base stations by thebase station. Additionally, the physical cell identities and the cellglobal identities of the cells served by the base station are re portedto at least one neighboring base station of the wireless communicationnetwork. Preferably, this can be done via an interface connecting thebase stations of the wireless communication network, e.g. X2 interfaceor an S1 interface, which are standard interfaces in 3GPP Long TermEvolution networks. If two different cells with different cell globalidentities have the same physical cell identity it is determined ifdetailed cell information is required. For example, two cells with thesame physical cell identity and different cell global identities can beserved by two neighboring base stations of the wireless communicationnetwork, the distance of the cells being big enough, i.e. the radiocoverage areas of these cells do not overlap, to avoid a conflict of thephysical cell identities. The detailed information about the cells canhelp determining if a conflict of physical cell identities exists.

The detailed cell information contains preferably the position and thesize of the cell, e.g. in terms of coverage area and main antenna lobeorientation. An unnecessary reassignment of at least one PCI can beavoided when the number of available PCIs is below a third threshold.Based on the detailed cell information a central network element and/orthe base stations of the cells with the same PCIs determines if areassignment of at least one PCI is necessary because of a PCI conflict.An unnecessary reassignment of at least one PCI is avoided when thenumber of available PCIs is below the third threshold and the detailedcell information indicates that the distance of the corresponding cellsis big enough because the low number of available PCI renders a newconflict of PCIs probable.

When the number of available PCIs is above the third threshold nodetailed cell in formation is required, a conflict of physical cellidentities is reported to the information algorithm in this case.

If detailed cell information is required a message is sent to the basestation serving the cell of that detailed cell information is requiredasking for the detailed cell information. Based on the detailed cellinformation it is determined if a physical cell identity conflict mayexist. If a physical cell identity conflict may exist the cell globalidentity search algorithm is executed. Alternatively, the detailed cellinformation is sent by the base station serving the cell which may havea physical cell identity conflict to the neighbour base station withouthaving asked detailed cell information. In this case the neighbour basestation is able to determine if a physical cell identity conflict mayexist. Finally, the detailed cell information can also be provided bythe base stations to the central network element which can decide on aphysical cell identity conflict.

In accordance with embodiments of the invention the method furthercomprises the detection of the loss of connections between the servingbase station and a plurality of mobile devices. A loss of connection isdetected when no uplink data is received without an initiated handoverto a neighboring base station of the wireless communication network.When the number of lost connections fulfills a third criterion aphysical cell identity conflict is reported to the information algorithmby the base station. Preferably, the third criterion is a fourththreshold of the number of lost connections between the base station andthe plurality of mobile devices.

In accordance with embodiments of the invention the method furthercomprises requesting an automatic neighbor relation measurement. Theautomatic neighbor relation measurement is performed by a mobile deviceof the wireless communication network. The mobile device reports atleast the physical cell identity and the cell global identity of thecell which has been specified by the serving cell. When a physical cellidentity conflict is detected in the automatic neighbor relationmeasurement by the first base station the physical cell identityconflict is reported to the information algorithm by the base station.

In accordance with embodiments of the invention the reassignment of atleast one physical cell identity is initiated by the central networkelement.

In accordance with embodiments of the invention the reassignment of atleast one physical cell identity is initiated by the base station.

In accordance with embodiments of the invention the base station sendsdetailed cell information to a different base station that has requestedthe detailed cell information.

In accordance with embodiments of the invention the base stationreceives detailed cell information from a different base station thathas sent the detailed cell informa tion.

In accordance with embodiments of the invention the central networkelement receives detailed cell information from base stations that havesent the detailed cell information.

In accordance with embodiments of the invention the central networkelement is an operation and maintenance node.

A person of skill in the art would readily recognize that steps ofvarious above-described methods can be performed by programmedcomputers. Herein, some embodiments are also intended to cover programstorage devices, e.g., digital data storage media, which are machine orcomputer readable and encode machine executable or computer-executableprograms of instructions, wherein said instructions perform some or allof the steps of said above-described methods. The program storagedevices may be, e.g., digital memories, magnetic storage media such as amagnetic disks and magnetic tapes, hard drives, or optically readabledigital data storage media. The embodiments are also intended to covercomputers programmed to perform said steps of the above-describedmethods.

BRIEF DESCRIPTION OF THE DRAWINGS

The description and drawings merely illustrate the principles of theinvention. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples recited herein are principally intended expressly to be onlyfor pedagogical purposes to aid the reader in understanding theprinciples of the invention and the concepts contributed by theinventors to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass equivalents thereof.

Some embodiments of methods and apparatus in accordance with embodimentsof the present invention are now described, by way of example only, andwith reference to the accompanying drawings, in which:

FIG. 1 shows a schematic view of a wireless communication network with aconflict of physical cell identities,

FIG. 2 shows a schematic view of a wireless communication network havinga conflict of physical cell identities,

FIG. 3 shows a block diagram of a method for avoiding conflicts ofphysical cell identities in a wireless communication network,

FIG. 4 shows a schematic view of a plurality of cells of a wirelesscommunication network where two cells have the same physical cellidentity,

FIG. 5 shows a schematic view of a plurality of cells of a wirelesscommunication network where a hotspot of a cell is located in anon-neighboring cell.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic view of five cells 100 of a wirelesscommunication network, each cell 100 having a different physical cellidentity from another cell 100. Further, FIG. 1 shows two neighboringcells 102, 103 of the wireless communication network having the samephysical cell identity. When a mobile device is located in the regionwhere the two cells 102, 103 overlap each other the conflict of physicalcell identities can lead to a loss of the connection between a basestation serving one of the two cells 102, 103 and the mobile device isnot able to decode the received information from the base station. Thus,the connection between the serving base station and the mobile device islost.

FIG. 2 shows a schematic view of five cells 100 of a wirelesscommunication network, each cell having a different physical cellidentity from each other. Two cells 102, 103 belong to the same wirelesscommunication network having the same physical cell identity. Thetransmission range of the two cells 102, 103 with the same physical cellidentity do not overlap each other. The cells 102 and 103 neighbor thecell 104 which has a different PCI.

Because of the conflict of the same PCIs of the cells 102, 103 ahandover procedure of a mobile device being located in cell 104 mayfail. For example, the handover request is sent from the base stationserving cell 104 to the base station serving cell 102 although thehandover should be performed to the neighbour cell 103. The decision ofthe serving base station 104 to send out a handover request is based onthe reported PCI from the mobile device. The handover request message issent to the wrong cell 102 because of the conflict of physical cellidentities. Thus, the base station serving the neighbour cell 103 towhich the handover should be performed is not prepared to communicatewith the mobile device. The base station serving cell 102 acknowledgesthe handover preparation procedure although the mobile device is not inthe transmission range from cell 102. As a result of this PCI conflict acontext release message is not sent by the base station serving cell 102to the base station serving cell 104 and the mobile device sends aconnection re-establishment message to the unprepared base stationserving neighbour cell 103.

FIG. 3 shows a block diagram of an embodiment of the invention. In step106 the number of missing context release messages 108 is compared witha first threshold. When the threshold is reached the cell globalidentity search algorithm 110 is executed in the serving cell. The errorrate of connection re-establishment messages 112 is compared in step 114with a second threshold. When the second threshold is reached the cellglobal identity search algorithm 110 is executed in the neighbour cell.Alternatively, the neighbour cell sends a failure indication message tothe serving cell to start the cell global identity search algorithm 110as well in the serving cell.

The cell global identity search algorithm 110 requests from mobiledevices the cell global identity of at least one cell with the physicalcell identity reported by the steps 106 and/or 114 to the cell globalidentity search algorithm 110 via an automatic neighbor relationmeasurement for this physical cell identity. If the cell global identitysearch algorithm 110 finds two different cell global identities of twodifferent cells with the same physical cell identity a conflict ofphysical cell identities is reported to the information algorithm 116.The information algorithm 116 informs a central network element 118and/or the neighbour base stations 120 serving the cells causing thephysical cell identity conflict.

The physical cell identities and the cell global identities of the cellsserved by the neighboring base stations 122 are received by the basestation in step 124. In step 124 additionally the physical cellidentities and the cell global identities of the neighbouring cells ofthe served cells of the neighbour base station are reported to the basestation of the wireless communication network.

In step 126 it is determined if detailed cell information is requiredwhen at least two cells have the same physical cell identity. Noadditional cell information is required in network configurations wherea sufficient number of physical cell identities is available to beassigned to one of the cells causing the physical cell identityconflict. If no detailed cell information is required the physical cellidentity conflict is reported to the information algorithm 116.

Detailed cell information is required when the number of availablephysical cell identities is under a third threshold so that areassignment of a physical cell identity could cause a new physical cellidentity conflict. For avoiding the reassignment of a physical cellidentity although there is no physical cell identity conflict, detailedcell information about the cells causing the physical cell identityconflict is requested. The detailed cell information can for examplecomprise the size and the position of the cell which can be provided interms of cell type or coverage area, coordinates and main antenna lobeorientation. The detailed cell information is exchanged between the basestations via an X2 or S1 interface. Based on the detailed cellinformation it is determined if the cell global identity searchalgorithm 110 is executed.

In step 128 lost connections 130 between the base station and mobiledevices are detected. Lost connections can be caused by a physical cellidentity conflict. For example, when two neighboring cells have the samephysical cell identity, see FIG. 1. If the number of lost connections130 reaches a fourth threshold the information algorithm 116 isexecuted.

An automatic neighbor relation measurement 132 is requested in step 134.The automatic neighbor relation measurement is performed by a mobiledevice that reports every physical cell identity and cell globalidentity in the transmission range of the mobile device to the basestation. When the same physical cell identity is assigned to two cellswith different cell global identities a physical cell identity conflictis reported to the information algorithm 116.

Every time the information algorithm 116 is executed the central networkelement 118 and the base stations serving the cells with the samephysical cell identities are informed about a conflict of physical cellidentities. When the central network element and the base stations areinformed about a conflict of physical cell identities the centralnetwork element may decide if a reassignment of a physical cell identityof the cells causing the physical cell identity conflict is performedfrom the central network element 118 or if the reassignment is performedby one of the base stations 120 serving the cells causing the physicalcell identity conflict.

FIG. 4 shows a schematic view of a plurality of cells 134 of a wirelesscommunication network comprising two cells with the same physical cellidentity 102 and 103. The two cells 102 and 103 are served by twoneighboring base stations 136 and 138. Although two neighboring basestations 136 and 138 serve two cells 102 and 103 with the same physicalcell identity it is not mandatory that there exists a conflict ofphysical cell identities because the distance between the cells 102 and103 can be big enough to avoid physical cell identity conflicts. If thenumber of available physical cell identities for a reassignment of thephysical cell identity of cell 102 or 103 is below a threshold, detailedcell information of the cells 102 and 103 is required to determine if aconflict of physical cell identities exists. If the number of availablephysical cell identities is above the threshold a new physical cellidentity can be assigned to cell 102 or 103 without requesting detailedcell information.

The detailed cell information comprises information of the size and theposition of the cell which can be provided in terms of cell type orcoverage area, coordinates and main antenna lobe orientation. Based onthis detailed cell information it is determined if a reassignment of aphysical cell identity of the cell 102 or 103 is necessary to avoid aphysical cell identity conflict. Usually this is the case for smallcells or for the deployment of many small cells in an environment oflarge overlay cells.

FIG. 5 shows a schematic view of a wireless communication network 134comprising two cells 102 and 103 with the same physical cell identity. Ahotspot 140 of the cell 103 is located in a neighboring cell 142 of cell102. The hotspot can be caused by real radio conditions in specialtopological environments. For example, the base station of cell 103 maybe located at a hillside such that a hotspot of the cell may begenerated on the other side of the valley at another hillside in cell142.

This hotspot may cause a conflict of physical cell identities althoughthe distance between the cells 102 and 103 seems to be big enough toavoid a conflict of physical cell identities.

This conflict of physical cell identities can be detected by analyzingthe handover procedures or the detection of lost connections with mobiledevices, see FIG. 3.

LIST OF REFERENCE NUMERALS

-   -   100 cell of the wireless communication network    -   102 cell with the same PCI as 103    -   103 cell with the same PCI as 102    -   104 cell located between 102 and 103    -   106 comparing 108 with first threshold    -   108 number of missing context release messages    -   110 cell global identity search algorithm    -   112 error rate of connection re-establishment messages    -   114 comparing 112 with second threshold    -   116 information algorithm    -   118 central network element    -   120 base station serving 102 and 103    -   122 physical cell identities and cell global identities of        neighboring base stations    -   124 receiving 122    -   126 detailed cell information    -   128 detecting 130    -   130 lost connections between a mobile device and a base station        of the wireless communication network    -   132 automatic neighbor relation measurement    -   134 plurality of cells of the wireless communication network    -   136 base station serving cell 102    -   138 base station serving cell 103    -   140 hotspot of cell 103    -   142 neighboring cell of cell 102

1. A computer implemented method of operating a base station of awireless communication network, wherein the method comprises: comparinga number of missing context release messages (108) with a firstcriterion; comparing the error rate of connection reestablishmentmessages (112) with a second criterion; executing a cell global identitysearch algorithm (110) if at least one of the first and the secondcriterion is fulfilled, wherein the cell global identity of at least onecell with the physical cell identity of the cell where the criterion wasfulfilled is requested via an automatic neighbour relation measurement,and wherein a conflict of physical cell identities is detected andreported to an information algorithm (116) when for the physical cellidentity at least two cell global identities are found; an informationalgorithm for informing a central network element (118) and/or at leastone base station (120) of the mobile communication network about aconflict of physical cell identities when a conflict of physical cellidentities is reported to the information algorithm; and initiating areassignment of at least one physical cell identity.
 2. The method ofclaim 1, wherein the method further comprises: receiving the physicalcell identities and the cell global identities of the cells served bythe neighbouring base stations (122) by the base station; reporting thephysical cell identities and the cell global identities of the cellsserved by the base station to at least one neighbouring base station ofthe wireless communication network; determining if detailed cellinformation (126) is required when two different cells with twodifferent cell global identities have the same physical cell identity;reporting a conflict of physical cell identities to the informationalgorithm, if no detailed cell information is required; asking for thedetailed cell information, if detailed cell information is required, bysending a message to the base station serving the cell of that detailedcell information is required determining based on the detailed cellinformation, if the cell global identity search algorithm is executed;and executing the cell global identity search algorithm, if the physicalcell identity conflict exists.
 3. The method of claim 1, wherein themethod further comprises detecting the loss of connections between thebase station and a plurality of mobile devices (130), whereby a loss ofconnection is detected when no uplink data is received without aninitiated handover to a neighbouring base station of the wirelesscommunication network, and wherein a physical cell identity conflict isreported to the information algorithm by the base station when thenumber of lost connections fulfills a third criterion.
 4. The method ofclaim 1, wherein the method further comprises requesting an automaticneighbour relation measurement (132), wherein a physical cell identityconflict is reported to the information algorithm by the base stationwhen the same physical cell identity is assigned to two cells withdifferent cell global identities.
 5. The method of claim 1, wherein thereassignment of at least one physical cell identity is initiated by thecentral network element.
 6. The method of claim 1, wherein reassignmentof at least one physical cell identity is initiated by the base station.7. The method of claim 1, wherein the base station requests detailedcell information of at least one different base station.
 8. The methodof claim 1, wherein the base station sends detailed cell information toat least one different base station that has requested the detailed cellinformation.
 9. The method of claim 1, wherein the first criterion is afirst threshold of missing context release messages per a time interval.10. The method of claim 1, wherein the second criterion is a secondthreshold of the error rate of connection reestablishment messages. 11.The method of claim 1, wherein the third criterion is a fourth thresholdof the number of lost connections between the base station and theplurality of mobile devices.
 12. The method of claim 1, wherein thecentral network element is an operation and maintenance node.
 13. Themethod of claim 1, wherein the detailed cell information is informationof the size and the position of the cell in terms of coverage area andmain antenna lobe orientation.
 14. A computer program product comprisingmachine executable instructions for performing the method of claim 1 ona base station.
 15. A base station apparatus being operable to perform amethod according to claim 1.